This application is based on Application No. 2001-265664, filed in Japan on Sep. 3, 2001, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an air-fuel ratio control device for an internal combustion engine and particularly concerns an air-fuel ratio control device for an internal combustion engine, by which an air-fuel ratio of air-fuel mixture supplied to the internal combustion engine is controlled so as to efficiently obtain the purifying performance of a catalytic converter.
2. Description of the Related Art
Conventionally, as one of air-fuel ratio control devices of an internal combustion engine, JP-A-H5-39741 discloses the following control device: in an internal combustion engine having a catalytic converter, an air-fuel ratio sensor is provided upstream of the catalytic converter and an O2 sensor is provided downstream of the catalytic converter, an air-fuel ratio on the upstream side is synchronized with the rotation of the internal combustion engine, a forcible oscillation value is reversed to a positive or negative value, a correction coefficient is updated such that a mean air-fuel ratio on the upstream side of the catalytic converter is set at a target air-fuel ratio, the median air-fuel ratio being detected by the air-fuel ratio sensor, when an air-fuel ratio on the downstream side of the catalytic converter is biased to a rich or lean side by the O2 sensor provided downstream of the catalytic converter, a target air-fuel ratio on the upstream side is corrected in a direction of canceling the bias to improve the purifying performance of the catalytic converter, during transient driving such as acceleration and deceleration, in which an irregular air-fuel ratio appears transiently, application of a forcible oscillation signal is prohibited, and degradation in exhausting characteristics is prevented.
However, in a conventional air-fuel ratio control device, forcible oscillation is prohibited only in transient driving, and in the other states forcible oscillation is always applied. Even in a relatively stable condition, an air-fuel ratio after the catalytic converter is biased due to interference such as introduction of purge. In this case (e.g., when being biased to a rich side), when application of forcible oscillation continues, a rich state other than a lean state exists. The lean state is a demanded air-fuel ratio from the state of the catalytic converter. Consequently, optimizing the state of the catalytic converter is interfered, resulting in deterioration in control response. In some cases, exhaust gas may be deteriorated in a rich state of forcible oscillation.
Further, immediately after returning from a fuel cutting state, the catalyst converter enters a state of excessive oxygen, and a purification factor of NOx is considerably reduced relative to a lean state provided upstream of the catalyst converter.
The present invention is devised to solve the above problems and has as its object the provision of an air-fuel ratio control device for an internal combustion engine, by which even in a state other than a transient state, when an O2 sensor provided downstream of a catalyst converter is in a rich state from a first predetermined value or in a lean state from a second predetermined value, periodic forcible oscillation is suspended, and a state for offsetting the biased state of the O2 sensor provided downstream of the catalyst converter is continued until the biased state is ended (until a lean state from the first predetermined value or a rich state from the second predetermined value is provided), so that control can be exercised only in a state required for optimizing the state of the catalyst converter, thereby improving response in control and eliminating the possibility of deteriorating exhaust gas.
Besides, the object of the present invention is to provide an air-fuel ratio control device for an internal combustion engine, by which forcible oscillation after returning to fuel cutting is controlled such that first rich side control time is corrected in an extending direction according to fuel cutting time, so that oxygen of a catalytic converter is consumed and a catalytic converter is immediately brought into a state of a good purification factor.
An air-fuel ratio control device for an internal combustion engine is provided with an air-fuel ratio sensor which is provided upstream of a catalytic converter provided in an exhaust system of the internal combustion engine and detects an air-fuel ratio of the internal combustion engine, an O2 sensor which is provided downstream of the catalytic converter and detects a concentration of oxygen after the catalytic converter, a reference air-fuel ratio target value setting means for setting a reference air-fuel ratio target value based on the number of revolutions and filling efficiency of the internal combustion engine, an O2 voltage target setting means for setting a target value of an output voltage of the O2 sensor based on the number of revolutions and filling efficiency of the internal combustion engine, an air-fuel ratio target value correcting means for obtaining an air-fuel ratio target value correction value based on an output voltage of the O2 sensor and a target value set by the O2 voltage target setting means, a forcible air-fuel ratio oscillation width target value correcting means for obtaining a forcible air-fuel ratio oscillation width target value based on the number of revolutions and filling efficiency of the internal combustion engine, an air-fuel ratio computing means for computing an air-fuel ratio target value based on outputs of the reference air-fuel ratio target value setting means, the air-fuel ratio target value correcting means, and the forcible air-fuel ratio oscillation width target value correcting means, an air-fuel ratio correction value computing means for computing a correction value based on an air-fuel ratio target value computed by the air-fuel ratio target value computing means and an output of the air-fuel ratio sensor, an injector driving time correction value computing means for obtaining a forcible air-fuel ratio oscillation width injector driving time correction value based on the number of revolutions and filling efficiency of the internal combustion engine, and an injector driving time setting means for setting time for driving an injector based on a correction value from the air-fuel ratio correction value computing means and a correction value from the injector driving time correction value computing means.
According to the above configuration, it is possible to exercise control simply by using a state required for optimizing a state of the catalytic converter, improve responsiveness of control, eliminate possibility of deteriorating exhaust gas, and immediately optimize the state of the catalytic converter even in a relatively stable condition.
An air-fuel ratio control device for an internal combustion engine may be characterized in that the forcible air-fuel ratio oscillation width target value correcting means forcibly varies the reference air-fuel ratio target value and the air-fuel ratio target value correction value to a rich side and a lean side in an alternate manner with predetermined widths in synchronization with the rotation of the internal combustion engine.
An air-fuel ratio control device for an internal combustion engine may be characterized in that for the forcible air-fuel ratio oscillation width target value correcting means, a forcible air-fuel ratio oscillation period setting means is provided which sets an air-fuel ratio oscillation period based on the number of revolutions of the internal combustion engine.
An air-fuel ratio control device for an internal combustion engine may be characterized in that for the forcible air-fuel ratio oscillation width target value correcting means, a forcible air-fuel ratio oscillation prohibiting means is provided which prohibits periodic forcible air-fuel ratio oscillation according to an output voltage of the O2 sensor. The forcible air-fuel ratio oscillation prohibiting means prohibits periodic forcible air-fuel ratio oscillation and continues a state for offsetting a detection state of an output voltage of the O2 sensor when an output voltage of the O2 sensor is at a first predetermined value or more or at a second predetermined value or less.
According to the above configuration, it is possible to improve accuracy of control and prevent deterioration of exhust gas.
An air-fuel ratio control device for an internal combustion engine may be characterized in that regarding forcible air-fuel ratio oscillation correction performed after returning to fuel cutting, correcting time of an initial rich side is corrected to an extending side according to fuel cutting time, in the forcible air-fuel ratio oscillation width target value correcting means.
According to the above configuration, it is possible to consume oxygen of the catalytic converter, bring the catalytic converter immediately into a state of a good purification factor, and immediately optimize the state of the catalytic converter.